There are many types of plate heat exchangers known to the applicant which are suitable for air to air heat and mass transfer, and reference can, for example, be made to the PCT application PCT/AU94/57097 (PCT/GB93/02633) standing in the name of Rose. However, tests which have been conducted by the applicant on heat exchangers which utilised a somewhat similar principle to that shown, have indicated that the heat exchange coefficient in terms of watts per square meter per degree Kelvin are likely to be in the range of from 30 to 40. These figures are much too low for heat exchangers when used with evaporative air coolers, since to achieve the desired heat and mass transfer the physical size and cost would both be too large.
Other relevant prior art known to the applicant include the PCT application PCT/GB90/00675 (WO 90/13784) standing in the name of the Secretary of State for Trade and Industry in the United Kingdom. A still further relevant heat exchanger as far as this invention is concerned is the subject of the U.S. Pat. No. 4,781,248 in the name of Pfeiffer (assignee Schmidt). However both the latter two heat exchangers appear to encounter problems related to pressure drop because both heat exchangers employ surfaces which will provide a high degree of turbulence. The high degree of turbulence is likely to provide a much higher heat exchange coefficient, but that is of little value in a heat exchanger when used in an evaporative air cooler, wherein power consumption is a matter of some concern.
Other relevant prior art on the aspect of humidifying return air will be found in U.S. Pat. No. 4,002,040 in the name of Munters, and U.S. Pat. No. 4,380,910 in the name of Hood. Prior art which is relevant to inclined air flow in heat exchangers include U.S. Pat. No. 4,781,248 in the name of Pfeiffer mentioned above, U.S. Pat. No. 5,050,671 in the name of Fletcher and U.S. Pat. No. 4,631,213 in the name of Bosne. It would also appear that the PCT Application No WO 88/01722 in the name of Fischer is relevant prior art, but probably to a lesser degree than the aforementioned prior art references.
It is well understood by those in the art that the heat exchange coefficient will improve if turbulence is increased, laminar flow substantially reducing the coefficient. Heat exchange coefficient is also known to be a function of velocity and increases with the fluid velocity. However, to disturb laminar flow by excessive turbulence requires input of excessive power, and will result in loss of efficiency as identified above. Associated with this characteristic, is change of air flow direction, and this also requires additional power input.
The main object therefore of this invention is to provide a simple and effective air to air heat exchanger of the contra flow type wherein there is sufficient turbulence to contribute to a relatively high heat exchange coefficient, but pressure drop is maintained very low so that excessive power input is not required.
A series of experiments conducted by the Applicant, and the study of computer models has indicated that the highest likely effectiveness is achieved by the stacked plate type of heat exchanger, and the highest effectiveness yet achieved in the prior art tested by the Applicants has been in a stack plate heat exchanger wherein the plates are provided with ribs or guides to form a plurality of channels which are of general U-shape and are arranged in a contra flow configuration. An effectiveness of 79 Watts per square meter per degree Kelvin at 1 meter per second was achieved with such a heat exchanger, but there was a limitation imparted by the configuration of the spaced U-shaped channels, in that air velocities in the different channels varied, and air direction was continuously changed through the heat exchanger.